1. Technical Field
The present invention relates to an optical recording medium and a recording and reproducing apparatus capable of recording or reproducing signals of high density by irradiating a plurality of thin recording films formed on a substrate with a high energy beam such as laser beam.
2. Background Art
Recently, commercialization of optical recording media capable of recording, erasing and reproducing information, and research and development of rewritable optical recording medium of high density capable of recording moving images of high quality are actively done. Known examples of rewritable optical recording media include the phase change optical recording medium having a disk-shaped substrate and an overlaying information layer such as a thin chalcogenide film using Te or Se as the base such as Ge—Sb—Te or In—Se, or a semimetallic thin film such as In—Sb. A magneto-optical recording medium having a thin metal film of Fe—Tb—Co or the like as an information layer is also known. Further, a write-once optical recording medium using a dye material is also known.
In the phase change optical recording medium, for example, an information layer composed of the phase change material is momentarily irradiated with a laser beam focused in a light spot of a sub-micron order size, and the irradiated area is locally heated up to a specified temperature. When heated over a crystallization temperature, the irradiated area is transited from amorphous state to crystalline state, and when further heated over a melting point to be fused and then quenched, it is transited to amorphous state. Either the amorphous state or the crystalline state is defined as recorded state or erased state (unrecorded state), and by forming in a pattern corresponding to the information signal, reversible information recording or erasing may be realized. The optical characteristic of crystalline state is different from the optical characteristic of amorphous state, and by making use of this difference, the signal can be reproduced by detecting optically as a reflectivity change or a transmissivity change.
In a magneto-optical recording medium, for example, a magneto-optical recording thin film is irradiated with a focused laser beam, and is locally heated to a specified temperature. By applying a magnetic field simultaneously when heating, the direction of magnetization of the magneto-optical recording thin film is reversed depending on the information, so that the information can be recorded or erased.
As a high density data recording method on an optical recording medium, mark length recording is known. In mark length recording, various mark lengths are recorded between various space lengths, and recorded information is assigned in both mark length and space length. In other proposed optical recording medium, in order to increase the capacity of the optical recording medium outstandingly, a plurality of information layers are provided, and a laser beam is irradiated from one side, and information is recorded or rewritten in each information layer.
When recorded in a phase change recording medium by the mark length recording method, the amorphous region is defined as a mark, and the crystalline region as a space. In order to record at higher density in this recording medium, the length of the marks and spaces to be recorded must be shortened. However, when the space length is shorter, the heat at the trailing edge of the recorded mark has an effect on temperature rise at the leading edge of the mark to be recorded next, or the heat at the leading edge of the mark recorded next has an effect on the cooling process of the preceding mark, that is, so-called heat interference occurs. By this heat interference, the edge position of the leading edge or trailing edge of the recording mark is deviated from the proper position, and the bit error rate is deteriorated on reproduction.
To solve this problem, for example, it has been proposed to prevent the mark edge position from varying due to heat interference due to recording by varying the leading edge position and trailing edge position of recorded pulse so that the space length space recording may be a specified length, when the space is shorter than the specified length on detecting the length of space between marks (Japanese Patent No. 2,679,596, corresponding U.S. Pat. No. 5,490,126).
However, when a plurality of information layers are provided, each information layer must be irradiated with a proper quantity of laser beam. For example, as shown in FIG. 11, in an optical recording medium having two information layers, that is, a first information layer 904 and a second information layer 908, it must be designed so that a laser beam 901 may be irradiated to the second information layer which is at the remoter side from the irradiation direction of the laser beam 901, and may be returned to the incident side to reproduce the information. Accordingly, the transmissivity of the laser beam 901 passing through a protection layer 903, the first information layer 904 and a protection layer 905 at the side closer to the incident side of the laser beam 901 is generally designed to be higher than about 50%. To obtain such transmissivity higher than 50%, the protection layer 903 and protection layer 905 which are thin film adjacent to the first information layer 904 are made of nonmetallic transparent layers. In order that the second information layer 908 positioned at the remoter side from the incident side of the laser beam 901 may have a greater quantity of reflected light, generally, a reflection layer 910 made of a metal material is disposed adjacently to the second information layer 908.
In such configuration, depending on whether there is a thin metal film adjacently to the information layer or not, a difference is made in the heating and cooling condition of the information layer. That is, if each layer is irradiated with a laser beam of same pulse condition, different recording marks are provided in layers, and if the recording mark edge is moved (edge shift) and information is reproduced between layers, jitter of reproduced signal increases in either information layer, thereby increasing the error rate.